1. Field of the invention
The present invention relates to a power source apparatus for microwave oven. More specifically, the present invention relates to a novel power source apparatus for microwave oven which is suitable for a commercial power source of a relatively high voltage, for example, 200 V or 220 V.
2. Description of the prior art
One example of an inverter circuit which can utilized as a power source apparatus for microwave oven is disclosed in Japanese Patent Laying-open No. 52-35903 (H05B6/66) laid open on Mar. 18, 1977 or Japanese Patent Laying-open No. 59-191290 (H05B6/68) laid open on Oct. 30, 1984. This prior art is a power source apparatus utilizing an inverter of a voltage resonance system, and a resonance time constant thereof is decided by leakage inductance of a high frequency transformer, inductance of primary and secondary windings of the high frequency transformer, and a resonance capacitor. In an inverter circuit of such a voltage resonance system, a fluctuation of an input voltage of a main circuit, that is, a fluctuation of a power source voltage of a commercial power source results in a fluctuation of a resonance voltage. Therefore, it is necessary to use a switching element having a large withstand voltage by taking such a fluctuation of the resonance voltage into account. For example, if the input voltage is 100 V, a peak voltage thereof becomes approximately 141 V (=100.sqroot.2), and therefore, a resonance voltage of approximately 600 V is applied to the switching element at a rated input voltage. Therefore, since the resonance voltage varies according to the fluctuation of the input voltage, in the above described case, a switching element having a rating of 900 V/50 A, for example, is used.
On the other hand, recently, it is proposed that a commercial power source of 200 V is utilized for obtaining a cooking apparatus such as a microwave oven with a large capacity, a high speed and etc. When the above described input voltage of the inverter circuit becomes 200 V, a switching element having a rating of approximately 1800 V/30 A is needed. However, a switching element which operates in a high frequency region for a power source apparatus for microwave oven and has such a large withstand voltage is not put into practical use. Therefore, the inverter circuit is disclosed in the Japanese Patent Laying-open No. 52-35903 or Japanese Patent Laying-open No. 59-191290 cannot be utilized as a power source apparatus for microwave oven which is driven by a commercial power source of 200 V.
Furthermore, in Japanese Patent Laying-open No. 2-101962 (H02M3/28, 3/335) laid open on Apr. 13, 1990, it is disclosed a half-bridge converter in which a series circuit of two switching elements, a series circuit of two resonance capacitors and a series circuit of two feed-back diodes are connected to a direct current power source, and a series circuit of a primary winding of a transformer and a first resonance inductor is connected between a connection point of the two switching elements and a connection point of the two resonance capacitors. A half-bridge converter disclosed this prior art is a circuit which is intended to be used for a stabilized power source apparatus mainly, and therefore, there is no proposal that such a half-bridge converter is utilized as a power source apparatus for microwave oven.
Although it is possible to consider that the half-bridge converter disclosed in the Japanese Patent Laying-open No. 2-101962 is utilized as a power source apparatus for microwave oven, the half-bridge converter of this prior art cannot be utilized as it is. More specifically, in the above described prior art, in order to control an output voltage of the half-bridge converter, a switching frequency of the two switching elements is controlled by an error voltage of an output voltage of a load and a reference voltage; however, a magnetron which is utilized in a microwave oven is a constant voltage load with respect to the half-bridge converter, and therefore, in order to control the output power of the magnetron, an output current is to be detected. In such a case, a current transformer for detecting the output current of the magnetron may be provided at a secondary circuit of a high frequency transformer and an output of the current transformer may be inputted to a control circuit. However, in a practical circuit, at a view point of safety, it is needed to isolate the secondary circuit of the high frequency transformer and the control circuit from each other, and therefore, it becomes necessary to use a current transformer having a large creeping distance and a large space distance. Therefore, a power source apparatus becomes large and high cost. In addition, in the half-bridge converter of the prior art, the resonance inductor is connected in series to the primary winding, and therefore, in order to implement a power source apparatus for microwave oven having a resonance frequency of 50 kHz and a magnetron output power of 800 W, for example, it is necessary to use a large resonance inductor having inductance of 10 .mu.H approximately. Therefore, in a case where the half-bridge converter disclosed in the Japanese Patent Laying-open No. 2-101962 is used as it is, a power source apparatus for microwave oven becomes large and high cost.